Magnetic cores used in motors, transformers, and the like are required to have high magnetic flux density and low iron loss. Conventionally, electrical steel sheets have been stacked in such magnetic cores, yet in recent years, dust cores have attracted attention as magnetic core material for motors.
The most notable characteristic of a dust core is that a 3D magnetic circuit can be formed. Since electrical steel sheets are stacked to form a magnetic core, the degree of freedom for the shape is limited. A dust core, on the other hand, is formed by pressing soft magnetic particles coated with insulation coating. Therefore, all that is needed is a die in order to obtain a greater degree of freedom for the shape than with electrical steel sheets.
Press forming is also a shorter process than stacking steel sheets and is less expensive. Combined with the low cost of the base powder, dust cores achieve excellent cost performance. Furthermore, since the surfaces of the electrical steel sheets are insulated, the magnetic properties of the electrical steel sheet in the direction parallel to the steel sheet surface and the direction perpendicular to the surface differ, causing the magnetic cores consisting of stacked electrical steel sheets to have the defect of poor magnetic properties in the direction perpendicular to the surface. By contrast, in a dust core, each particle is coated with insulation coating, yielding uniform magnetic properties in every direction. A dust core is therefore appropriate for use in a 3D magnetic circuit.
Dust cores are thus indispensable material for designing 3D magnetic circuits, and due to their excellent cost performance, they have also been used in recent years from the perspectives of reducing the size of motors, reducing use of rare earth elements, reducing costs, and the like. Research and development of motors with 3D magnetic circuits has thus flourished.
When manufacturing high-performance magnetic components using such powder metallurgy techniques, there is a demand for components to have excellent iron loss properties after formation (low hysteresis loss and low eddy current loss).
In response to this demand, JP 4630251 B2 (PTL 1) and WO08/032707 (PTL 2) disclose techniques for improving magnetic properties as follows. Iron-based powder is adjusted so that upon sieve classification with a sieve having an opening of 425 μm, the iron-based powder that does not pass through the sieve constitutes 10 mass % or less, and upon sieve classification with a sieve having an opening of 75 μm, the iron-based powder that does not pass through the sieve constitutes 80 mass % or more, and so that upon inspecting at least 50 iron-based powder cross-sections, measuring the grain size of each iron-based powder, and calculating the grain size distribution including at least the maximum grain size, crystal grains with a grain size of 50 μm or more constitute 70% or more of the measured crystal grains.
JP H08-921 B (PTL 3) discloses a technique related to pure iron powder for powder metallurgy with excellent compressibility and magnetic properties. The impurity content of the iron powder is C≤0.005%, Si≤0.010%, Mn≤0.050%, P≤0.010%, S≤0.010%, O≤0.10%, and N≤0.0020%, and the balance of the powder consists substantially of Fe and incidental impurities. The particle size distribution is, on the basis of weight percent by sieve classification using sieves prescribed in JIS Z 8801, constituted by 5% or less of particles of −60/+83 mesh, 4% or more to 10% or less of particles of −83/+100 mesh, 10% or more to 25% or less of particles of −100/+140 mesh, and 10% or more to 30% or less of particles passing through a sieve of 330 mesh. Crystal grains included in particles of −60/+200 mesh are coarse crystal grains with a mean grain size number (a smaller number indicating a larger grain size) of 6.0 or less measured by a ferrite grain size measuring method prescribed in JIS G 0052. When 0.75% of zinc stearate is blended as a lubricant for powder metallurgy and the result is compacted with a die at a compacting pressure of 5 t/cm2, a green density of 7.05 g/cm3 or more is obtained.
Furthermore, JP 2005-187918 A (PTL 4) discloses a technique related to insulation-coated iron powder for dust cores such that an insulating layer is formed on the surface of iron powder particles having a micro Vickers hardness Hv of 75 or less, and JP 2007-092162 A (PTL 5) discloses a technique related to high compressibility iron powder that includes by mass %, as impurities, C: 0.005% or less, Si: more than 0.01% to 0.03% or less, Mn: 0.03% or more to 0.07% or less, S: 0.01% or less, O: 0.10% or less, and N: 0.001% or less, wherein particles of the iron powder have a mean crystal grain number of 4 or less and a micro Vickers hardness Hv of 80 or less on average.